3 Aortic Valve and Aortic Root
Anatomy and Function
Key Points
• Aortic root is a dynamic system that maximizes blood flow through the aortic valve (AV) with no structure fixed in position or dimension during the cardiac cycle.
• The AV lacks a true anatomic annulus (ring-shaped); instead, it is supported by a crown-shaped base at the aortoventricular junction.
• Named for the coronary artery arising from the corresponding sinus of Valsalva, the three AV cusps include the anterior right coronary cusp (RCC), noncoronary cusp (NCC) near the interatrial septum (IAS), and left coronary cusp (LCC) associated with the left atrium (LA).
• Normal AV cusps are soft, pliable, unequal in size (NCC > RCC > LCC) and have geometric dimensions that ensure competent AV function (Figure 3-1).
• Sinuses of Valsalva are three outpouches of the aorta that give rise to the coronary ostia, the upper portions of which join with the superior AV cusps at the aorta in the nonlinear sinotubular junction (STJ).
Normal Aortic Valve and Root Anatomy and Function
• The aortic root is a three-dimensional structure composed of the aortic annulus, AV cusps, sinuses of Valsalva, STJ, and proximal ascending aorta (see Figure 3-1).
• Cusp attachment forms the hemodynamic junction between the left ventricle (LV) and the aorta. The proximal structures are subjected to left ventricular (LV) pressure (diastolic expansion and systolic contraction) whereas distal structures exposed to aortic pressures expand during systole, facilitating AV opening.
• The aortic root is joined to the LV (see Figure 3-1) with 45% of its circumference attached to ventricular muscle (muscular interventricular septum [IVS] and LV free wall) and 55% to fibrous tissue (aortomitral membrane and membranous IVS).
• Each cusp free margin has a central thickening, the nodule of Arantii, and on either side, the lunula, which form an overlapping closure line (or commissure) during diastole.
Congenital Anomalies of Cusp Number
• Congenital absence of the AV is very rare; abnormalities in the number of AV cusps can range from unicuspid to hexacuspid.
• These valves can be regurgitant or stenotic at birth or develop functional impairment later in life.
Bicuspid Aortic Valve
• In children, a BAV may be stenotic at birth without calcification, whereas in adults, it is from superimposed calcification.
• Characteristic midesophageal (ME) AV short axis (SAX) and long axis (LAX) transesophageal echocardiography (TEE) views (see Case 3-1 on the companion website) show two slightly thickened cusps with two commissures, a raphe, an elliptical systolic orifice and an asymmetrical closure line.
• Possible coexisting anomalies with a BAV can be identified by echocardiography: coarctation of the aorta, post-stenotic aortic dilatation, aortic dissection and patent ductus arteriosus (PDA).
• RCC and LCC (80%) fusion form larger anterior and smaller posterior cusps with both coronary arteries arising from the anterior cusp. RCC and NCC (20%) fusion has a larger right than left cusp with one coronary artery arising from each cusp. Fusion of the LCC and NCC is rare.
Unicuspid and Quadracuspid Aortic Valve
• Unicuspid AV is rare and consists of two types: either acommissural with a central orifice or, more commonly, a posterior commissure with an eccentric elliptical-shaped orifice (Figure 3-2).
• Quadracuspid AV is the rarest and appears as four equal cusps (see Figure 3-2). It may have associated coronary anomalies and present with aortic regurgitation (AR).
Echocardiographic Imaging of the Aortic Valve and Root
• Echocardiography documents complete anatomic and functional assessment of the AV, including cusp morphology, mobility, coaptation and aortic root size.
• Continuous wave (CW) Doppler alignment is best in the transgastric (TG) views with the spectral trace used to assess antegrade (AS) and retrograde (AR) flow (Table 3-1).
TABLE 3-1 BEST VIEWS FOR ASSESSING THE AORTIC VALVE
AR, aortic regurgitation; AV, aortic valve; AVA, aortic valve area; LAX, long axis; LCC, left coronary cusp; LVOT, left ventricular outflow tract; ME, midesophageal; NCC, noncoronary cusp; RCC, right coronary cusp; RVOT, right ventricular outflow tract; SAX, short axis; 3D, three-dimensional; TEE, transesophageal echocardiography; TG, transgastric; TTE, transthoracic echocardiography; 2D, two-dimensional.
Aortic Stenosis
Key Points
• Obstruction can occur at the valvular level, below the valve (fixed, dynamic), or above the valve.
• Important associated findings include mitral regurgitation (MR), left ventricular hypertrophy (LVH) and aortic root dilation. There is often significant diastolic dysfunction.
• Severe MR or low cardiac output (CO) reduces the transaortic flow rate, causing a low gradient even with severe AS and obliges aortic valve area (AVA) calculation. Falsely low gradients can also occur in the presence of a high ejection fraction with a low stroke volume (SV).
• High CO and low systemic vascular resistance (SVR) falsely elevate pressure gradients, which may misdiagnose severe AS as with a dynamic left ventricular outflow tract (LVOT) obstruction.
Etiology of Aortic Stenosis
Step 1: Determine the Etiology
• Obstruction can occur below, above, or at the AV level. It is easily differentiated based on the site of increased velocity seen with color or spectral Doppler and local anatomy.
• Valvular AS is a common valvular lesion caused by congenital (bicuspid 38%), rheumatic (24%), or degenerative calcification in the elderly (33%).
• Supravalvular stenosis is uncommon arising from a congenital condition, such as Williams’ syndrome with persistent or recurrent obstruction in adulthood.
Features of Valvular AS
Calcific AS is typified by calcification within the central part of each cusp without commissural fusion, resulting in a stellate-shaped systolic orifice (Figure 3-6).
Rheumatic AS is characterized by commissural fusion with thickening and calcification along the cusp edges resulting in a triangular systolic orifice, almost always accompanied by rheumatic mitral valve (MV) changes (see Figure 3-6).
Associated findings with AS (Figure 3-7) are important to identify including MR, LVH with variable ventricular function, and aortic root dilatation. Their presence may affect the type and timing of intervention.
Quantitative Assessment of Aortic Stenosis
Step 2: Assess AS Jet Velocity
• CW Doppler parallel alignment with flow direction (or within 15 degrees) may be difficult using TEE. It aims to record the highest AS jet velocity, which is the strongest predictor of clinical outcome.
• The gray scale spectral Doppler signal is optimized with a velocity scale to fit the signal, a 100-mm/s time scale, high wall filters, and reduced gain to record a smooth velocity curve with a dense outer edge and clear maximum velocity (Figure 3-8).
• Usually, three or more beats are averaged in sinus rhythm, and at least five consecutive beats are mandatory with irregular rhythms using care to avoid post-extrasystolic beats.
• The CW Doppler velocity curve shape distinguishes the level, severity and dynamics of obstruction (see Figure 3-8).
Step 3: Assess Transaortic Pressure Gradient
• CW Doppler quantifies AS by measuring blood velocity across the AV and calculating the transaortic pressure gradient (ΔP) using the simplified Bernoulli equation: ΔP = 4V2.
• The simplified Bernoulli equation assumes that the proximal velocity can be ignored. This is a reasonable assumption when it is less than 1 m/s, but if greater than 1.5 m/s or the aortic velocity is less than 3.0 m/s, it is included when calculating maximum gradients: ΔP = 4(V2max − V2proximal).
• The mean pressure gradient has potential advantages over the peak gradient determined from the peak velocity. It is obtained by averaging instantaneous gradients during the entire systole and not from the mean velocity (see Figure 3-8).
Pitfalls: Factors Affecting the Pressure Gradient
• Underestimation of aortic velocity causes an even greater underestimation in pressure gradients, owing to the squared relationship between velocity and pressure.
• Pressure gradient underestimation occurs with high SVR, low CO and MR, whereas overestimation occurs with low SVR, high CO and AR.
• Peak Doppler pressure gradients represent the maximum instantaneous pressure difference across the aortic valve. This pressure is typically greater than the unphysiologic peak-to-peak LV and aortic pressures, which do not occur at the same time point at cardiac catheterization.
• Measures of AS severity (Vmax, mean gradient, AVA) remain accurate when severe AR accompanies AS but will be higher than expected for a given AVA.
Pressure Recovery
• PR is the increase in pressure (decrease in velocity) that occurs shortly after the vena contracta as kinetic energy is reconverted into potential energy.
• PR is greatest from a small orifice (native AV or prosthesis) with gradual distal widening, compared with abrupt widening to the larger aorta yielding a lower PR (Figure 3-9).
• In AS, PR (in mm Hg) is related to the effective orifice area (EOA)-to-aortic area (AoA) ratio; a small AoA (diameter < 30 mm) favors PR. The initial pressure drop across the stenosis from LV to vena contracta by Doppler may be significantly higher (overestimate) than the relevant actual net pressure drop (see Figure 3-9).
Left Ventricular Systolic Dysfunction
• LV systolic dysfunction with severe AS reduces the AS velocity and pressure gradient despite a small AVA. This is a condition termed low-flow low-gradient AS, commonly defined by
• Dobutamine stress echocardiogram alters stroke volume (or ejection fraction [EF]) and changes aortic velocity, mean gradient, and AVA as flow rate increases. This helps differentiate these clinical situations.
1 LV systolic dysfunction from severe AS results in a moderate transaortic peak velocity and mean pressure gradient and a small AVA usually with improvement in LV function after aortic valve replacement (AVR).
• Stress echocardiography is not indicated in a patient with a low EF and resting AS velocity of 4.0 m/s or mean gradient of 40 mm Hg. This represents a normal LV response to high afterload, not a poor LV, and will improve after relief of stenosis.
Step 4: Calculate Valve Area
• Planimetry of the anatomic AVA is tracing the smallest orifice and can be technically challenging to obtain using TEE (Figure 3-10).
• Careful TEE probe manipulation locates the smallest orifice at maximal cusp separation in the ME AV LAX view that, after angle rotation to a ME AV SAX view, identifies this orifice with all cusps and a circular aortic wall visualized.
• Doppler velocity and pressure gradients are flow dependent. Calculation of the functional AVA is helpful when flow rates are very low or very high because the AVA remains constant despite the flow across the valve.
• Functional AVA is calculated based on the continuity equation, which represents the equal SV ejected through the LVOT and stenotic AV:
• Volume flow rate through any cross-sectional area (CSA) is equal to the CSA times flow velocity over the ejection period (VTI). This equation can be rewritten as
Grading Aortic Stenosis
Step 5: Assessing Aortic Stenosis Grading
TABLE 3-2 LIMITATIONS AND IMPLICATIONS IN ASSESSING AORTIC STENOSIS
| Limitations | Implications | |
|---|---|---|
| Etiology | ||
| AS jet velocity (CW Doppler) | ||
| AS pressure gradient (CW Doppler) | ||
| Velocity ratio (VR = VLVOT/VAV) | ||
| AVA planimetry Anatomic AVA | ||
| AVA continuity Measures EOA |
AS, aortic stenosis; AV, aortic valve; AVA, aortic valve area; CO, cardiac output; CW, continuous wave; EOA, effective orifice area; LV, left ventricular; LVH, left ventricular hypertrophy; LVOT, left ventricular outflow tract; MR, mitral regurgitation; SVR, systemic vascular resistance.
From Baumgartner H, Hung J, Bermejo J, et al. Echocardiographic assessment of valve stenosis: EAE/ASE recommendations for clinical practice. J Am Soc Echocardiogr. 2009;22:1-23.
Step 5a: No Discrepancies in Aortic Stenosis Quantification
• General guidelines (Table 3-3) have been established by the American College of Cardiology/American Heart Association (ACC/AHA) and European Society of Cardiology (ESC) for categorizing AS severity as mild, moderate, or severe to provide guidance for clinical decision making.
• If during cardiac surgery for other reasons TEE reveals evidence of AS, quantitation may be attempted, but if the intercept angle is inadequate, epiaortic scanning should be performed (Figure 3-11).
Aortic Regurgitation
Key Points
• Advanced techniques of quantitation may be difficult in the operating room (OR), where AR is usually assessed by measurement of the vena contracta, pulsed Doppler of the descending aorta and the secondary effects on other structures (e.g., LV).
• The degree of jet penetration into the LV is unreliable in the quantitation of AR. The pressure half-time (PHT) of the AR jet is extremely dependent on the LV compliance.
